The present invention relates to a hetero junction field effect transistor (FET), and particularly to a semiconductor device in which the recessing process is facilitated by inserting a barrier layer immediately below a gate and which enables improvement in the voltage resistance of the gate and trans-conductance.
GaAsFETs are of general interest as devices that exhibit excellent high-speed and high-frequency characteristics. Schottky gate FETs (MESFET) among this type of devices have been brought into practical use.
As shown in FIG. 6, such a MESFET comprises a buffer layer 23 and an active layer 25 which are formed on a semi-insulating GaAs substrate 21 and on which are formed a source electrode 11, a gate electrode 15 and a drain electrode 13. Both the source electrode and the drain electrode are brought into ohmic contact with the active layer 25 and the gate electrode 15 is brought into Schottky contact with the semiconductor in the active layer 25, thus producing a potential barrier which allows a depletion layer to extend in the semiconductor. The cross-sectional area of the channel is changed by controlling the thickness of the potential barrier using the gate voltage so that the current between the source and drain electrodes is controlled. The buffer layer 23 is provided for the purpose of reducing the effect of the crystallinity of the substrate.
FIG. 7 shows a high-mobility transistor (HEMT) comprising an undoped GaAs layer 33, a donor impurity-doped N-type Al.sub.x Ga.sub.1-x As layer 35 and a contact layer 37 which are provided in turn on a semi-insulating GaAs substrate 31. Electrons fall from the N-type Al.sub.x Ga.sub.1-x As layer 35 into the GaAs layer 33 to form a two-dimensional electron gas 33e in the vicinity of the hetero-junction interface which is spatially separated from the ionized donor serving as a scattering source, resulting in the achievement of high mobility owing to a reduction in the Coulomb scattering effect.
SIS (Semiconductor Insulator Semiconductor) FETs in which neither the barrier layer nor the active layer (shown in FIG. 7) are doped have also been proposed, as well as.hetero junction FETs.
However, a GaAs FET such as shown in FIG. 6, as well as AlGaAs hetero buffer FETs (not shown), has the problem of exhibiting low gate voltage resistance because of the direct Schottky contact with the GaAs active layer and poor high-frequency characteristics because the trans-conductance cannot be greatly increased (for example, 1.5 dB at 12 GHz).
The HEMT shown in FIG. 7 (or reverse HEMT) also has a problem in that the concentration of the carriers flowing through the channel is limited because the active layer portion is not doped, and thus the trans-conductance cannot be increased.
Both ISFETs and hetero junction FETs also involve the problem that trans-conductance cannot be increased because the barrier layer and the active layer are not doped.